Cellular communication systems are known. Such systems are, typically, comprised of a number of cells, each having a service coverage area, and a number of cellular telephones (communication units). The service coverage areas of adjacent cells may be arranged to partially overlap in such a manner as to provide a substantially continuous coverage area in which a communication unit receiving service from one cell may be handed off to an adjacent cell with no interruption in service. The Groupe Special Mobile (GSM) Pan-European digital cellular system, as specified in GSM recommendations available from the European Telecommunications Standards Institute (ETSI) and incorporated herein by reference, is an example of just such a system.
A cell's radio coverage is provided by a base transceiver station (BTS). Each BTS may contain one or more transceivers (TRX) which can simultaneously receive on one frequency and transmit on another. Communication between a BTS and a mobile communication unit (or mobile station) (MS) typically occurs using a portion of a pair of frequencies (transmit and receive) temporarily assigned in support of the communication transaction at the BTS.
The pair of frequencies assigned for use at the BTS are typically referred to as a radio channel. Downlink transmissions (from BTS to MS) on the radio channel occur on a first frequency of the pair of frequencies. Uplink transmissions (from MS to BTS) on the radio channel occurs on the second frequency of the pair of frequencies.
The GSM system is a time division multiplex/time division multiple access (TDM/TDMA) system providing eight full duplex signal paths (8 TDM slots per TDM frame) on each radio channel. A single, primary radio channel assigned to a BTS, by virtue of its being time multiplexed, can support up to seven full rate duplex traffic users (speech or data) in addition to a multiplexed common control channel within the eight TDM slots. Additional, secondary radio channels assigned to the same cell can provide a full complement of eight full rate traffic users (in the 8 TDM slots) per radio channel, since the control channel within the primary radio channel can control allocation of communication resources on secondary radio channels.
Transmissions (control or speech and/or data traffic) from a BTS to an MS, on the downlink, occupy a first TDM slot (downlink slot) on a first frequency of a radio channel and transmissions from a communication unit to a BTS, on the uplink, occupy a second TDM slot (uplink slot) on the second frequency of the radio channel. The uplink slot on the second frequency is displaced in time three TDM slot positions following the downlink slot on the first frequency. The uplink slot on the second frequency is offset 45 MHz lower in frequency than the downlink. The uplink slot and downlink slot (together providing a two-way signal path for a single user) may be referred to as a "communication resource", allocated by the BTS to an MS for exchanging signals. The term "communication resource" also typically includes an associated signalling channel, as for example the GSM specified slow associated control channel used with traffic channels.
Exchanges of paging and setup control information within GSM between MSs and BTSs typically occurs on the common control channel (CCCH) which occupies at least one slot of a primary channel of the BTS. Transmitted by the BTS on the CCCH are distinctive identification signals as well as synchronization and timing information common to all other frequencies and slots of the BTS. CCCH information allows an MS to differentiate between primary and non-primary channels.
System control attributes of a GSM-like system are quite complex and demanding. The operation is hinged to the existence of a primary channel being present for each BTS. Key system control functions such as cell selection and handover are based upon the primary channel. Mobiles select a serving BTS based upon signal measurements of primary channels of nearby BTSs. Handovers are achieved, in part, based upon primary channel signal measurements performed by a mobile and transferred to a serving BTS.
Upon activation, an MS scans a set of frequencies in search of CCCH identification signals transmitted from proximate BTSs. Upon detecting a CCCH identification signal the communication unit measures a signal quality factor (such as signal strength and/or bit error rate) of the identification signal as a means of determining relative proximity of the BTS. Upon completing the scan of frequencies within the set, the MS generally selects the BTS providing the largest relative signal quality factor, as a serving BTS. Upon identifying, and locking onto a suitably strong signal (and registering if necessary) the communication unit monitors the selected CCCH for incoming calls. Should the communication unit desire to initiate a call, an access request may be transmitted using the CCCH of the serving BTS.
During normal operation (including during active calls), the MS monitors for, identifies, and measures primary channels of nearby BTSs. If involved in an active call, the MS relays measurement information back to the base site on a slow associated control channel (SACCH). Through such a process, it is possible for the MS to maintain an association with the most appropriate BTS. During an inactive state the process may entail an autonomous switching by the MS to a different BTS, causing perhaps a re-registration by the MS with the system indicating that such a switch has occurred. Alternatively, during an active communication exchange, the MS may be commanded by the system to handover to a more appropriate BTS.
Access by an MS to a local BTS may allow the MS telephony access to a communication target, such as another MS, served by the same, or another BTS, or to a subscriber within a public switched telephone network (PSTN). Access by the MS to a local BTS may also provide the MS access to a diversity of other data services.
In general, communication access is provided to the MS through a cellular infrastructure system which, in the case of a PSTN target, may include the BTS, a base station controller (BSC), a mobile switching center (MSC), and the PSTN network. Under GSM, a BSC may control a number of BTSs. An MSC, connected to the PSTN network, may control a number of BSCs.
The exchange of information within the infrastructure network of the GSM cellular system generally occurs over high speed (2.048 mb/s) transmission links, based upon CCITT standardized exchange protocol. Physical mediums used to facilitate these high speed links include wireline, coax, fiber optic, or microwave. Such links may be utilized, for example, to interconnect remote BTSs with a controlling BSC.
The use of high speed transmission links within the cellular infrastructure has been justified, in the past, by high capacity requirements due, in part, to the relatively large geographic areas covered by BTSs. High speed links, in other cases, have been justified by the highly variable nature of communication traffic through BTSs and by a desire for standardization.
While, in the past, high speed transmission links have worked well, the cost has not been justified in all applications. In some cases the cost of a high speed transmission link between a BTS and BSC is not justified by the communication traffic. BTSs serving rural areas may be lightly loaded and spaced at relatively large distances thereby increasing cost in areas least likely to need the capacity of such links. There is often also some difficulty providing such conventional links due to certain logistical problems. (e.g. a remote BTS site at the top of a light pole).
In urban areas, on the other hand, as cellular traffic has increased, the trend has been to divide cells into increasingly smaller cells (microcells). The use of microcells reduces the average distance between MS and BTS allowing for reduced transmission power (thereby increasing the viability of hand-held portables). Reducing average transmission power of communication exchanges allows for a greater number of users within a system by allowing closer reuse of communication resources.
Increasing the capacity of the system by reducing cell size may reduce the average traffic through microcell BTSs. Use of high speed links to interconnected BSCs with microcell BTSs, in such cases, may result in a significant mismatch of need to capacity with sever economic consequences.
As with the use of high speed links at the BTS, it has likewise been traditional in the past to include the capability for radio test diagnostic subsystems at each BTS site. The test subsystem provides MS emulation and is used, under control of an operations and maintenance center (OMC), to test the radios of the BTS, thereby insuring the integrity of the BTS and, through loopback testing, the overall network. The test subsystem, in the past, has been limited to internal test routines within the BTS and has been provided through the high speed link.
Because of the importance of microcells and of cellular service in rural areas, with little communication traffic, a need exists for an alternative to the high speed data links typically used to interconnect remote BTS sites to the remainder of the cellular infrastructure. Such an alternative should have the potential for working equally well in rural areas as in the microcell environment of urban or other relatively high traffic areas. Such an alternative should also provide a convenient and economical means of testing remote BTS sites.